1. Field of the Invention
The present Invention relates to a sub-muffler utilizing basalt fiber, which has an excellent sound absorbing property and an excellent heat resistance as a sound absorbing material.
2. Description of the Related Art
An automobile muffler is used as a member to absorb exhaust sound. Fiberglass is used today as a sound absorbing material of a muffler. Recently, the necessity of energy saving and the tighter control on exhaust emission have brought about a rise in the automobile engine temperature, which is accompanied by a rise in the exhaust gas temperature at the muffler portion up to 800° C. or higher. For this reason, there is an urgent need to develop a sound absorbing material for mufflers with a higher heat resistance (capable of dealing with a temperature range from 750° C. to 900° C.).
For example, exposure to a high-temperature (up to 800° C.) gas, emitted from the exhaust system of various types of engine, brings about a decrease in the durability and the sound absorbing property of a sub-muffler equipped with a commercially available fiberglass mainly composed of a synthetic raw material of SiO2—Al2O3—CaO and the like (hereinafter referred to as a sub-muffler (1)). The reason for this decrease is as follows. Fibers in the commercially available fiberglass mainly composed of a synthetic raw material of SiO2, Al2O3 and the like stick together when the fibers are exposed to an exhaust gas of a temperature up to 800° C. This results in an increase in the apparent filament diameter of the fiberglass, and the increase in the diameter, in turn, causes a decrease in the flexibility of the fiberglass as a sound absorbing material. Eventually, the sound absorbing property of such fiberglass decreases.
In addition, exposure to a high-temperature (up to 830° C.) gas, emitted from the exhaust system of various types of engine, brings about a decrease in the durability and the sound absorbing property of a sub-muffler equipped with a commercially available fiberglass mainly composed of synthetic raw materials of SiO2—Al2O3—MgO and the like (hereinafter referred to as a sub-muffler (2)). The reason for this decrease is as follows. The commercially available fiberglass mainly composed of synthetic raw materials of SiO2, Al2O3 MgO and the like is crystallized by an exhaust gas at a temperature up to 830° C., which results in the loss of the flexibility of the fiber.
Furthermore, a temperature of 650° C. or higher of the exhaust system of various types of engine brings about a decrease in the durability and the sound absorbing property of a sub-muffler equipped with a fiberglass of synthetic raw materials other than those used in the sub-mufflers (1) and (2), specifically a fiberglass of synthetic raw materials of SiO2—Al2O3—CaO—Na2O system, SiO2—CaO—Na2O system, SiO2—Na2O system and SiO2—K2O system (hereinafter referred to as a sub-muffler (3)). The reason for this decrease is as follows. Since these types of fiberglass contain a considerable amount of alkali metal oxides such as CaO, Na2O and K2O, exposure to an exhaust gas at 650° C. or higher facilitates crystallization, sticking and corrosion by the acid components of various types in the exhaust gas.
Besides, the manufacturing cost of the sub-muffler (2) is high, specifically, at the level of several thousand yen per kilogram. The reason is as follows. In manufacturing the fiberglass of SiO2, Al2O3, MgO and the like from these oxides, firstly each of the oxides is purified and separated from the corresponding various raw materials to obtain the synthetic raw material. Secondly, these components are ground, and then the resultant powders are mixed. Subsequently, the mixture is melted at a high temperature, and thus the molten materials are formed into fiber.
Each of the sub-mufflers (1) and (3) has a complicated manufacturing process and has a heavy weight in order to keep the fiberglass at a lower temperature. This is because two steps are required for this purpose, such as a first step of winding SUS wool around the outer circumference of the inner pipe and a second step of winding a sound absorbing material of fiberglass around the circumference thereof.
In addition, the cost in the recycling process of the sub-mufflers (1) and (3) is, high. The burning of the fiberglass used in the sub-mufflers (1) and (3) in an incineration generates oxides each of which has a low melting point. The corrosion of refractory lining of the incinerator caused by the oxides decreases the durability of the incinerator. This requires an increase in the capital investment in plant and machinery.
Furthermore, use of the sub-mufflers (1), (2) and (3) has a difficulty in reducing the environmental burden as seen from the view point of Life Cycle Assessment (LCA). The manufacture of fiberglasses used respectively in the sub-mufflers (1), (2) and (3) generates more CO2 than otherwise, because they are produced by, firstly, synthesizing their own component oxides, secondly mixing the powders of the oxides in a certain predetermined composition, and then melting the respective mixtures.
In contrast, basalt continuous fiber made from a raw material of natural basalt raw stone is significantly less expensive than conventional continuous fiberglass. The basalt continuous fiber, however, has the following problem when used at a high temperature ranging from approximately 750° C. to approximately 900° C. A crystalline phase is generated from glass component, and this leads to problems such as the loss of flexibility and the separation at the boundary between the crystalline layer and the glass layer.
The facts that have been described thus far leads to the unavailability of fiberglass for absorbing sound, nor materials for heat-insulation parts for vehicles, which has a satisfactorily high heat resistance and a satisfactorily high durability, and which can be manufactured at a low cost, at the same time.
JP Patent Publication (Kokai) No. 10-77823 A discloses an invention of a muffler in which a fiber as a sound absorbing material is provided by winding around an exhaust pipe. In the disclosed invention, a continuous fiber is provided by winding around the outer circumference of the exhaust pipe where vent-holes are bored. The continuous fiber is provided by winding around at the positions where the vent-holes are located. The continuous fiber is provided by winding not in a state of being woven, but in a state of being layered, for the purposes of forming the fiberglass layers inexpensively and preventing the layers from clogging by oily substances in the exhaust gas. In this disclosure, such fibers as a fiberglass, a fiber made from basalt stone and a ceramic fiber made from high-purity silica, are cited as examples of the continuous fiber. Nevertheless, in the above disclosure, there is no indication at all, as to what kind of basalt fiber is used.